Using the technique of separating rat pups from their biological mothers shortly after birth and rearing the offspring in isolation, the experimenters are engaged in studies designed to determine the effects of maternal separation on several behavior and physiological variables. One series of proposed studies will be designed to determine which characteristics of the mother's physical presence and behavior are essential in preventing the cardiac response to maternal separation. Maternal characteristics will be systematically varied to determine their role in maintaining sympathetic nervous system functioning preweanling pups. A second series of studies will be conducted to separate the influences of length of maternal separation from age at which separation occurred as these affect the emotional development of infant rats. A third series of studies will be undertaken to more fully assess the ability of the rat mother to reduce the effects of stress during infancy. Maternally separated and nonseparated pups will be subjected to noxious stimulation during infancy. The influence of these experiences will be assessed in active-avoidance learning situations later in life.